Decoherence Puzzle in Measurements of Photons Originating from Electron-Positron Annihilation

S. Sharmaa, b, c, D. Kumara, b, c, P. Moskala, b, c, P. J-PET Collaboration aFaculty of Physics, Astronomy, and Applied Computer Science, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland bTotal-Body Jagiellonian-PET Laboratory, Jagiellonian University, Łojasiewicza 11, 30-348 Kraków, Poland cCenter for Theranostics, Jagiellonian University, Kopernika 40 St, 31-501 Kraków, Poland

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The entanglement of photons originating from electron-positron annihilation has not been experimentally proven. owever, the independent experiments performed so far unanimously confirm that the correlation between the linear polarizations of back-to-back photons from electron-positron annihilation is consistent with the assumption that these photons are entangled in the polarization. Yet, unexpectedly, recent experiments differ as regards the correlation of polarization direction of back-to-back photons after the decoherence induced by the scattering of one of these photons on the electron in the scattering material. In one of the experiments, the correlation before and after the decoherence of the photon state is the same, and in the other experiment, the scattering of one photon leads to a significant decrease in this correlation. Here we discuss this puzzle. Decoherent states were ensured provided by forcing one of the annihilation photons to scatter earlier before measuring the polarization correlation based on Compton kinematics. The comparison is made between the experimental setups used for the different measurements, and the results obtained are briefly discussed, highlighting the parameters that are important in performing such measurements. Finally, the main features of the J-PET detector are presented, along with the schemes for performing similar studies, so that the conclusive results can be used as remarks to solve the puzzle in question. Solving the decoherence puzzle will have crucial consequences for basic studies of entanglement, as well as for the proposed application of the photon polarization in positron emission tomography. If the correlation of the polarization of back-to-back photons from the electron-positron annihilation is the same before and after the scattering of these photons, then it will not be useful for the reduction of scatter fraction in positron emission tomography diagnostics.

DOI:10.12693/APhysPolA.142.428 topics: quantum entanglement, positron emission tomography (PET), Compton polarimeters, plastic scintillators